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Keywords:

  • metal-on-metal;
  • hip resurfacing;
  • retrievals;
  • wear;
  • metal ions

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES
  7. Supporting Information

Suboptimal component position and design are thought to lead to edge wear and raised blood metal ion levels in metal-on-metal hip resurfacing (MOM-HR). These factors are thought to influence the “contact patch to rim distance” (CPRD), and calculation of this distance may improve prediction of wear and blood metal ion levels. We measured blood cobalt and chromium ion levels and the wear rates of the bearing surfaces in 165 MOM-HR retrieval cases. We then determined the contribution and effect sizes of cup inclination and version angles, component size and design, and CPRD (calculated from case-specific data) on blood metal ion levels and component wear rates. Acetabular orientation explained between 16.3% and 28.5% of the variation in wear rates and metal ion levels, whereas component size and design explained between 7.3% and 21.8% of the variability. In comparison, CPRD explained up to 67.7% of the variability, significantly greater than any other variable (all p < 0.0001). CPRD is a good predictor of wear and improves our understanding of wear performance and the mechanisms leading to edge loading. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:167–174, 2014.

High revision rates have been widely reported for metal-on-metal hip resurfacing (MOM-HR),[1] and subsequently the use of MOM has become a major public health concern subject to widespread coverage in both the scientific and lay press. High revision rates have been partly attributed to the release of metal debris from the bearing surfaces. High wear, occurring as a result of edge loading, has been associated with devastating inflammatory soft tissue reactions.[2-4] These patients have poorer outcomes after revision surgery.[5]

Previous studies reported a significant association between cup inclination angle and both wear rates and blood metal ion levels.[6-8] However, the correlations with cup inclination are only weakly positive and therefore can not alone explain all the variation in wear rates and ion levels.[6, 8] Indeed, edge loading in MOM-HR is likely attributable not only to component position, but also component design[9, 10] and patient activity patterns.[11]

The Articular Surface Replacement (ASR; DePuy Orthopaedics) is widely recognized as having the poorest clinical and wear performance of the contemporary MOM-HR designs.[1, 12, 13] Design features such as the reduced “arc of cover” (the angle subtended by the articular surface of the cup) and head-cup clearance are thought to increase the likelihood of edge loading and high wear.[12, 13] An overview of the design variations among manufacturers is shown in Table 1.

Table 1. Design Specifications of the MOM-HR Devices Included in this Study
ProsthesisRadial Clearance (μm)Arc of Cover (°)Manufacturing Method (as-Cast, or Wrought)
Adept resurfacing system (Finsbury Orthopaedics)86.37160As cast
ASR hip replacement (DePuy Orthopaedics)49.47146–152As cast (plus heat treatment)
Birmingham hip resurfacing (Smith and Nephew)105.10159–163As cast
Durom™ hip resurfacing (Zimmer)97.67165Wrought
Cormet hip resurfacing system (Corin)68.23159–165As cast (plus heat treatment)

Many of the variables that affect wear, both surgical and design, likely do so by moving the contact area between the femoral and acetabular bearing surfaces closer to the cup rim. This distance has been described in the literature , and in this study we have termed it the ‘contact patch to rim distance’ (CPRD). As shown by the two-dimensional schematic (Fig. 1), a reduction in this distance increase the likelihood of edge loading and high wear.

image

Figure 1. 2D schematic illustrating key surgical- and design- specific variables contributing to the contact patch to rim distance (CPRD).

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No previous studies have reported the relative contributions or effect sizes of the relevant surgical and design variables (including CPRD) on wear or ion levels. Calculating CPRD will likely reflect the complex 3D geometric interaction of multiple variables, and calculating this for individual patients may improve wear prediction, allowing clinicians to better stratify their patients for long-term follow-up.

There were two parts to our study. First, we developed a geometric model to calculate CPRD and determined the contribution of five surgical and design variables to this measure. Second, we calculated patient-specific CPRDs for 165 MOM-HR retrieval cases. We then determined the contribution and effect size of CPRD and other commonly reported clinical and design variables, on wear rates and ion levels. Finally, we determined the predictive value of a low CPRD for raised ion levels and high wear rates.

METHODS

  1. Top of page
  2. ABSTRACT
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES
  7. Supporting Information

Calculating the Contact Patch to Rim Distance

A program was written in MATLAB (The Mathworks, Inc., Natick, MA) to calculate CPRD. Our model incorporated five variables: (1) cup inclination angle, (2) cup version angle, (3) arc of cover, (4) femoral head diameter, and (5) head-cup clearance. To calculate CPRD, we first considered there to be two directional vectors: (1) the cup orientation through the pole and (2) the joint reaction force. To create the cup orientation vector, we defined a vector passing vertically through the pole that had not been rotated. This was then transformed according to rotations defined by the radiographic definition of cup inclination and version.[15] The joint reaction force vector was created according to previously published data describing its direction for common patient activities[16]; For the purpose of this study, we used the joint reaction force attributed to the standing position.[16] In accordance with greater circle theory, the shortest distance between two points on a sphere is in the plane that passes through the sphere's center. As such, the vector dot product can be calculated between the two directional vectors, giving the angle between them in a plane that passes through the center of the head and cup. As the angle between them is known, the distance between them can be calculated, thus giving the distance from the joint reaction force (center of the contact patch) to the rim of the cup. To obtain the distance from the edge of the contact area to the rim (CPRD), the radius of the contact area must be calculated. Hertzian theory has been validated to calculate the radius of the contact area for MOM hip replacements.[17] The head-cup clearance is an important determinant to the size (radius) of the contact area. The radius of the contact area is then subtracted from the distance between the reaction force vector (center of the contact area) and the rim. Further subtractions are then incorporated into the model to account for differences in head diameter and arc of cover.

We ran our program, altering each contributing variable in isolation to calculate the mean change in CPRD for a 10-unit increase in that variable. In addition, we calculated the theoretic threshold cup orientation for each MOM-HR design included in this study at which the CPRD would be zero and the hip would edge load in the standing position.

Patients and Components

For the second part of the study we input prospectively collected clinical data into our model to calculate the individual CPRD for a series of retrieval cases. One hundred sixty-five consecutive MOM-HR cases (330 retrieved components) collected at our retrieval laboratory met our inclusion criteria. To be included, patients were required to have undergone full pre-revision clinical assessment. This included whole blood sampling to measure Co and Cr ion levels, and CT scanning to measure acetabular orientation, both according to previously described methods.[18] Sufficient pre-, intra-, and post-operative data were required to diagnose the reason for revision according to categories set out by the National Joint Registry for England and Wales.[1] For the purpose of wear and metal ion analysis, all cases were required to be unilateral hip replacements and to have been implanted for >12 months.[19]

Demographic data and details of the retrieved components are given in Table 2. Five designs were included: Adept Resurfacing System, Finsbury Orthopaedics (n = 7); ASR, DePuy Orthopaedics (n = 46); Birmingham Hip Resurfacing (BHR), Smith and Nephew (n = 73); Cormet Hip Resurfacing System, Corin (n = 30); and Durom Hip Resurfacing, Zimmer (n = 9).

Table 2. Summary of the Patient Demographic Data and Implant Details
 NumberMeanMedianRange
  1. a

    32% male patients, 68% female patients.

  2. b

    66% cups edge loaded, 34 cups not edge loaded.

Gender (male/female)52/113a
Age at primary surgery (years)545622 to 83
Time to revision (months)434012 to 121
Femoral head diameter (mm)474636 to 58
Angle of acetabular inclination (°)484815 to 82
Angle of acetabular version (°)1817−34 to 61
Arc of cover (°)158160145 to 165
Diametric clearance (μm)170190100 to 210
Contact patch to rim distance (mm)98−4 to 25
Whole blood cobalt (ppb)2870 to 387
Whole blood chromium (ppb)2050 to 179
Cup wear rate (μm/year)2750 to 90
Head wear rate (μm/year)940 to 25
Edge loaded (yes/no)109/56b

Wear Measurement

Measurements of the bearing surfaces were carried out using a Talyrond 365 (Taylor Hobson, Leicester, UK) roundness measuring instrument, specifically designed for high accuracy surface measurement of circular and cylindrical components. Components were mounted on a rotating air spindle (max. run out of 20 nm) and were centered and leveled with respect to the spindle access. A series of measurement traces were taken using a 2 mm ruby stylus (resolution: 10 nm). For each bearing couple, 36 polar and circumferential measurements were taken and analyzed using a previously described protocol.[7] Median (range) is reported in Table 2.

Statistical Methods

All analyses were performed using Stata/IC 12.1 (StatCorp, College Station, TX), and a p-value <0.05 was considered significant. For the second part of the study we performed separate analyses for each of four outcome variables: cup wear rate; head wear rate; Co level; and Cr level. Logarithmic transformations of all variables were necessary to improve the distribution of the models residuals and to ensure the assumption of constant variance in regression analyses.

We used simple and multiple linear regression statistics to determine associations between the four outcome variables and the five surgical and design predictor variables incorporated in our model to calculate patient-specific CPRD. Further linear regression statistics were then used to determine the association between CPRD itself and the four outcome variables. R2 statistics are presented for each fitted model.

Multiple linear regression models were then used to assess the simultaneous contribution of the predictor variables found to be significant from the univariable analyses and determine which variables best predicted outcome. Nonsignificant variables were eliminated using a process of backward elimination; at each stage the least significant predictor was eliminated from the model (which was then re-fitted) until all remaining variables were significant.

For each regression analysis we also reported the estimated effect sizes (EXP(β)) and associated 95% confidence intervals (CI). These quantify the relative change in the outcome variable for a unit increase in the associated predictor variable (for clearance, the estimated effect sizes were based on a 0.01 increase in clearance due to the small scale of measurement). Effect sizes >1 indicate a positive correlation and relative increase in the outcome variable for a unit increase in the predictor variable (i.e., an estimated effect size of 1.2 indicates that a unit increase in the predictor variable will lead to a 20% increase in the outcome variable). An effect size <1 indicates a negative correlation and relative decrease in the outcome variable for a unit increase in the predictor variable.

Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) for various CPRDs were calculated for high ion levels (≥7 ppb) and high wear rates (>5 μm/year).

RESULTS

  1. Top of page
  2. ABSTRACT
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES
  7. Supporting Information

Part 1: Contribution of Surgical- and Design-Specific Variables on CPRD

In order of decreasing effect on CPRD, the variables were: cup inclination angle, cup version angle, arc of cover, femoral diameter, and clearance (Table 3). For each MOM-HR design, the theoretical threshold ranges for cup orientation at which the CPRD will be 0 (i.e., the hip will be edge loading) is shown in Figure 2. The geometric effect of the five variables is shown in Supplementary figures (S-Figs. 1–4).

Table 3. Effect of a 10-Unit Change in Each Variable on the Contact Patch to Rim Distance
 Change in VariableChange in CPRD (mm)
Femoral diameter−10 mm−3.00
Head-cup clearance−10 μm−0.12
Arc of cover−10°−1.75
Cup inclination angle+10°−3.05
Cup version angle+10°−2.25
image

Figure 2. Threshold cup orientations at which a 50 mm diameter MOM-HR of each design will edge load (i.e., have a CPRD ≤ 0) in the standing position. The box (dotted line) represents Lewinnek's “safe zone” for cup implantation.

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Part 2: Contribution and Effect Size of Surgical- and Design Specific Variables, Including CPRD on Wear Rates and Metal Ion Levels

The results of the univariable and multiple linear regression analyses are given in Tables 4-8. From univariable analyses, inclination and version of the cup explained between 16.3% and 28.5% of the variation in wear rates and ion levels. In comparison, design variables (head diameter, clearance, and arc of cover) explained between 7.3% and 21.8% of the variability.

Table 4. Results of the Univariable and Multiple Linear Regression Analyses for Cup Wear Rate
 βEst. Effect (EXP (β))95% CIp-ValueR2 (%)
  1. Arc of cover, cup inclination, and cup version were the only independent predictors of cup wear rate.

  2. a

    R2 for the final multiple linear regression model = 33.6%.

Univariable associations
Femoral diameter (mm)−0.0850.9190.862–0.9780.0084.3
Radial clearance (μm)−0.0650.9370.884–0.9930.0292.9
Arc of cover (°)−0.1100.8960.855–0.940<0.00111.5
Cup inclination (°)0.0491.0501.032–1.068<0.00116.7
Cup version (°)0.0621.0641.036–1.093<0.00111.8
Final multiple regression model
Arc of cover (°)−0.1000.9050.867–0.944<0.00133.6a
Cup inclination (°)0.0461.0471.016–1.066<0.001 
Cup version (°)0.0401.0411.030–1.0630.001 
Table 5. Results of the Univariable and Multiple Linear Regression Analyses for Head Wear Rate
 βEst. Effect (EXP (β))95% CIp-ValueR2 (%)
  1. Arc of cover, cup inclination, and cup version were the only independent predictors of head wear rate.

  2. a

    R2 for the final multiple linear regression model = 29.2%.

Univariable associations
Femoral diameter (mm)−0.0750.9280.885–0.9720.0025.8
Radial clearance (μm)−0.0510.9500.909–0.9930.0243.1
Arc of cover (°)−0.0880.9160.884–0.949<0.00112.9
Cup inclination (°)0.0511.0521.031–1.077<0.00113.6
Cup version (°)0.0271.0281.014–1.042<0.0019.2
Final multiple regression model
Arc of cover (°)−0.0780.9250.895–0.956<0.00129.2a
Cup inclination (°)0.0361.0371.017–1.056<0.001 
Cup version (°)0.0241.0241.012–1.037<0.001 
Table 6. Results of the Univariable and Multiple Linear Regression Analyses for Whole Blood Cobalt
 βEst. Effect (EXP (β))95% CIp-ValueR2 (%)
  1. Arc of cover and cup inclination were the only independent predictors of blood cobalt.

  2. a

    R2 for the final multiple linear regression model = 16.4%.

Univariable associations
Femoral diameter (mm)−0.0480.9530.898–1.0110.1071.6
Radial clearance (μm)−0.0280.9720.921–1.0270.3160.6
Arc of cover (°)−0.0600.9410.899–0.9850.0104.1
Cup inclination (°)0.0381.0391.022–1.056<0.00111.9
Cup version (°)0.0371.0381.012–1.0650.0054.9
Final multiple regression model
Arc of cover (°)−0.0640.9380.899–0.9790.00416.4a
Cup inclination (°)0.0391.0401.023–1.056<0.001 
Table 7. Results of the Univariable and Multiple Linear Regression Analyses for Whole Blood Chromium
 βEst. Effect (EXP (β))95% CIp-ValueR2 (%)
  1. Arc of cover and cup inclination were the only independent predictors of blood chromium.

  2. a

    R2 for the final multiple linear regression model = 17.7%.

Univariable associations
Femoral diameter (mm)−0.0560.9450.896–0.9970.0392.6
Radial clearance (μm)−0.0220.9780.931–1.0290.3900.5
Arc of cover (°)−0.0690.9330.896–0.9730.0016.3
Cup inclination (°)0.0331.0341.019–1.049<0.00110.9
Cup version (°)0.0361.0361.013–1.0610.0035.4
Final multiple regression model
Arc of cover (°)−0.0720.9310.895–0.968<0.00117.7a
Cup inclination (°)0.0341.0351.020–1.050<0.001 
Table 8. Univariable Associations Between CPRD and Wear Rates/Blood Metal Ion Levels
 βEst. Effect (EXP (β))95% CIp-ValueR2 (%)
CPRD/cup wear rate−0.2280.7960.772–0.821<0.00167.7
CPRD/head wear rate−0.1620.8510.830–0.872<0.00160.9
CPRD/blood cobalt−0.1700.8430.815–0.873<0.00147.7
CPRD/blood chromium−0.1530.8580.831–0.885<0.00146.5

From the final multiple linear regression models the most significant independent predictors of wear rates and ion levels were cup inclination angle, the arc of cover, and cup version angle. The final models were able to explain between 16.4% and 33.6% of the variability in wear rates and ion levels. Neither head diameter nor clearance were deemed significant independent predictors in any of the final models.

CPRD was negatively correlated with cup and head wear rates and ion levels (Table 8 and Figs. 3-5). Overall CPRD explained a greater (all p < 0.001) proportion of the variability in wear rates and ion levels than any single variable or the final regression models. For instance, CPRD explained almost 70% (R2 = 0.68) of the variability in cup wear rates. In comparison, the final multiple regression model explained 34% (R2 = 0.34) of the variability in cup wear rate and cup inclination alone only 17% (R2 = 0.17). Similar results were seen for head wear rate and Co and Cr ion levels. The estimated effect size of CPRD on the four variables ranged between 0.796 and 0.858. Therefore a single unit decrease in CPRD (1 mm increase), would increase ion levels/component wear rates by ∼15–20%.

image

Figure 3. XY scatter plot showing the correlation between cup wear rate and CPRD.

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image

Figure 4. XY scatter plot showing the correlation between whole blood Co and CPRD. Cases in the shaded area represent those with blood metal ion levels lower than 7 ppb.

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image

Figure 5. XY scatter plot showing the correlation between whole blood Cr and CPRD. Cases in the shaded area represent those with blood metal ion levels lower than 7 ppb.

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The predictive value of low CPRD for raised ion levels (>7 ppb) and high wear rates (>5 μm/year) is summarized in Table 9. Low CPRD was a better predictor of bearing surface wear rates than it was of blood metal ion levels.

Table 9. The Predictive Value of a Low CPRD for Raised Blood Metal Ion Levels (>7 ppb) and High Bearing Surface Wear Rates (>5 μm/year)
CPRD (mm)CobaltChromiumCup Wear RateHead Wear Rate
Sens. (%)Spec. (%)PPV (%)NPV (%)Sens. (%)Spec. (%)PPV (%)NPV (%)Sens. (%)Spec. (%)PPV (%)NPV (%)Sens. (%)Spec. (%)PPV (%)NPV (%)
  1. We report sensitivity (sens), specificity (spec), positive predictive value (PPV) and negative predictive value (NPV) for a range of CPRDs.

<565.496.594.674.669.189.982.580.975.798.998.383.673.192.086.083.6
<672.890.788.178.073.182.073.182.081.193.590.986.079.186.079.186.0
<775.387.284.779.074.678.069.482.183.489.386.187.482.183.076.487.4
<880.380.279.381.279.171.064.683.586.582.880.088.585.177.071.388.5
<981.573.374.280.882.166.061.884.690.777.276.491.090.971.367.492.3
<1084.062.868.080.685.156.056.484.997.370.072.097.097.062.562.197.0

DISCUSSION

  1. Top of page
  2. ABSTRACT
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES
  7. Supporting Information

Our study contributes to understanding the wear performance of MOM-HR. To our knowledge, this is the first to quantify the contribution and effect size of specific surgical and design variables on bearing surface wear rates and blood metal ion levels from retrievals, and also the first to report the associations with CPRD as calculated from individual patient and implant data.

A previous multivariate analysis of MOM hip retrievals identified the presence of edge loading as the main predictor of bearing surface wear rates.[7] Other studies identified the angles of cup inclination[7] and version[20] as significant predictors of wear, and speculated that design variables, such as arc of cover[9, 10] and clearance,[14] may explain the variation in wear performance among MOM-HR designs. These variables are all thought to influence wear performance in the same way, by affecting the susceptibility of the hip to edge loading. Edge loading occurs when the contact area between the head and cup intersects the rim of the cup. Therefore, it is likely that CPRD can determine the susceptibility of a MOM-HR to edge loading and is likely to correlate well with component wear and blood metal ion levels.

Using our 3D model to calculate CPRD, we evaluated the relative contributions of five variables on the risk of edge loading. Surgical position of the cup, a combination of inclination and version angles, was the most important determinant of CPRD. The head diameter contributed significantly to CPRD, and this may help explain why small diameter MOM hips (i.e., those used in females) have higher revision rates.[1] Of the component design variables, “arc of cover” contributed more to CPRD than clearance.

We then showed that surgical positioning of the cup also contributed more to high wear rates and metal ion levels than differences in component design. Combined, cup inclination and version explained an average of 21% of the variability in wear rates and ion levels. In comparison, the design variables tested (head diameter, clearance, and arc of cover) explained on average 14%. Although this supports inclination as the most important variable associated with high wear, our results suggest that both suboptimal position and component design contribute significantly. It is clear that cup inclination alone is a poor predictor of bearing surface wear and blood metal ion levels.

Of the design features investigated, reduced arc of cover was most significantly associated with increased wear rates and higher ion levels, supporting evidence in the literature that the ASR hip resurfacing device, with a relatively shallow arc of cover, is significantly higher wearing than other designs.[12, 13] From our theoretical CPRD calculations for different MOM-HR designs, we showed how the ASR is more susceptible to suboptimal cup position. Even standing, a patient with a 50 mm diameter ASR hip resurfacing with a cup implanted within Lewinnek's “safe zone,”[21] may be subject to edge loading (Fig. 2). Optimal cup position is likely design specific.

In a similar study, Underwood et al.[14] described low clearance to be a significant risk factor for edge loading and high wear. However, our results do not support this, with clearance accounting for <5% of the variation in wear rates and ion levels. While in theory lower clearance does increase the size of the contact area and therefore the risk of edge contact, the variation in clearance among designs is small and not likely significant. Low clearance also provides improved distribution of contact pressures.

CPRD explained up to 70% of the variability in wear rates and ion levels, significantly more than for any other variable. Cup inclination, which is widely recognized as the most important determinant of edge loading,[8] only accounted for up to 17% of the variability in these outcomes. Our results show how the 3D interaction of multiple component position and design variables combine to account for a large portion of the variation in wear rates and ion levels. Our work builds on that by Underwood et al.[14] who found that edge loaded retrievals had a significantly lower mean “contact patch edge to rim (CPER) distance.”

Although CPRD explained a significant proportion of the variability in wear, ∼30–50% remained unexplained. This may be due in part to variation in activity patterns and suboptimal positioning of the femoral component. Mellon et al.[11] previously described an association between edge loading and patient-specific motion patterns. Although likely to be less influential than cup position, femoral position variables, such as horizontal offset and version likely affect the risk of edge loading. Although a previous study found no association between femoral version and wear rates,[22] this remains poorly understood. Future work may focus on extending our CPRD model to include femoral positional variables. Additionally, some of the variation in wear rates may be due to other causes of edge loading such as impingement[23] or micro-separation.[24] Both mechanisms may cause edge wear in the presence of an acceptable CPRD, particularly anterior impingement, which is more likely to occur in MOM HRs due to the retention of femoral bone and if the cup is implanted with a shallow inclination angle or a neutral/negative version angle.

CPRD was more strongly correlated with wear rates than ion levels. Some patients with high wearing hips likely developed symptoms limiting their activity. While they had a highly worn hip (and low CPRD), their limited activity prior to revision surgery may have facilitated clearance of Co and Cr ions from the blood. Our results show that low CPRD can predict high wear in MOM-HR. However, further evaluation of the predictive value of low CPRD for raised ion levels and clinical failure in a large prospective cohort of patients is essential.

We acknowledge limitations to this study. First, like all retrieval studies our results are not representative of the entire MOM population. However, we investigated a large number of cases, and the numbers of each hip design reflected the market distribution in the period during and prior to collection.[1] The variation and number of designs is unlikely to contribute significant bias. We investigated design features such as “arc of cover” and clearance in isolation and as part of the CPRD model. In combination with the use of multiple regression analyses we could reduce the confounding effect of multiple designs and identify design features associated with high wear. While CPRD accounts for many differences in component design, one aspect that was not investigated was the role of manufacturing variations, such as heat treatment, on wear. Whilst this is the most comprehensive model described in the literature to calculate the CPRD, we acknowledge that it is likely incomplete, and future work may identify other variables that can be incorporated in the model. An advantage of our method is the use of 3D CT scanning to measure cup inclination and version. In large diameter MOM hips, measuring cup position (particularly version) from plain radiographs is challenging due to the large metal head obscuring the margins of the cup.[25]

This study has quantified the relative contributions of surgical and design variables on the wear performance of MOM-HR, and demonstrated how CPRD can predict high wear in MOM-HR. This work contributes to our understanding of failure and may help facilitate the development of safer implant designs. These results are clinically significant given the large worldwide burden of patients implanted with MOM-HR devices over the past two decades and the continued use of well-performing resurfacing designs such as the BHR in selected patients.

REFERENCES

  1. Top of page
  2. ABSTRACT
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES
  7. Supporting Information
  • 1
    National Joint Registry for England and Wales. 9th Annual Report 2012.
  • 2
    Glyn-Jones S, Roques A, Taylor A, et al. 2011. The in vivo linear and volumetric wear of hip resurfacing implants revised for pseudotumor. J Bone Joint Surg Am 93:21802188.
  • 3
    Hart AJ, Sabah S, Henckel J, et al. 2009. The painful metal-on-metal hip resurfacing. J Bone Joint Surg Br 91-B:738744.
  • 4
    Langton DJ, Joyce TJ, Jameson SS, et al. 2011. Adverse reaction to metal debris following hip resurfacing: the influence of component type, orientation and volumetric wear. J Bone Joint Surg Br 93-B:164171.
  • 5
    Grammatopolous G, Pandit H, Kwon YM, et al. 2009. Hip resurfacings revised for inflammatory pseudotumour have a poor outcome. J Bone Joint Surg Br 91-B:10191024.
  • 6
    De Haan R, Pattyn C, Gill HS, et al. 2008. Correlation between inclination of the acetabular component and metal ion levels in metal-on-metal hip resurfacing replacement. J Bone Joint Surg Br 90-B:12911297.
  • 7
    Hart AJ, Muirhead-Allwood S, Porter M, et al. 2013. Which factors determine the wear rate of large diameter metal-on-metal hip replacements? Multivariate analysis of two hundred and seventy six components. J Bone Joint Surg Am 95:678685.
  • 8
    Morlock MM, Bishop N, Zustin J, et al. 2008. Modes of implant failure after hip resurfacing: morphological and wear analysis of 267 retrieval specimens. J Bone Joint Surg Am 90:8995.
  • 9
    Griffin WL, Nanson CJ, Springer BD, et al. 2010. Reduced articular surface of one-piece cups: a cause of runaway wear and early failure. Clin Orthop Rel Res 468:23282332.
  • 10
    Jeffers JRT, Roques A, Taylor A, et al. 2009. The problem with large diameter metal-on-metal acetabular cup inclination. Bull NYU Hosp Jt Dis 67:189192.
  • 11
    Mellon SJ, Grammatopolous G, Andersen MS, et al. 2013. Individual motion patterns during gait and sit-to-stand contribute to edge loading risk in metal-on-metal hip resurfacing. Proc I Mech Eng H 227:799810.
  • 12
    Langton DJ, Jameson SS, Joyce TJ, et al. 2010. Early failure of metal-on-metal bearings in hip resurfacing and large diameter total hip replacement: a consequence of excess wear. J Bone Joint Surg Br 92-B:3846.
  • 13
    Underwood R, Matthies A, Cann P, et al. 2011. A comparison of explanted articular surface replacement and Birmingham hip resurfacing components. J Bone Joint Surg Br 93:11691177.
  • 14
    Underwood RJ, Zografos A, Sayles RS, et al. 2012. Edge loading in metal-on-metal hips: low clearance is a new risk factor. Proc I Mech Eng H 226:217226.
  • 15
    Murray DW. 1993. The definition and measurement of acetabular orientation. J Bone Joint Surg Br 75-B:228232.
  • 16
    Bergmann G, Deuretzbacher G, Heller M, et al. 2001. Hip contact forces and gait patterns from routine activities. J Biomech 34:859871.
  • 17
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Supporting Information

  1. Top of page
  2. ABSTRACT
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES
  7. Supporting Information

Additional supporting information may be found in the online version of this article at the publisher's web-site.

FilenameFormatSizeDescription
jor22459-sm-0001-SupFig-S1.tif86KFigure S1. The effect of cup inclination and cup version on CPRD.
jor22459-sm-0002-SupFig-S2.tif73KFigure S2. The effect of “arc of cover” on CPRD.
jor22459-sm-0003-SupFig-S3.tif71KFigure S3. The effect of component size on CPRD.
jor22459-sm-0004-SupFig-S4.tif60KFigure S4. The effect of head-cup clearance on CPRD.

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